The region of acceptable duality in the space of operating parameters of a
coating process is called coating window. Their limits are set by coating d
efects. For the slot-coating process the low-flow limit is important. It co
rresponds to the maximum web speed at a given film thickness, or the minimu
m film thickness at a given web speed, at which the coating bead remains st
able. The available viscocapillary model is based on the Landau-Levich equa
tion, which is limited to small Capillary and Reynolds numbers. Under these
conditions, the minimum film thickness that can be coated decreases with d
ecreasing coating speed but many coating processes do not occur at low Capi
llary numbers. It is important to determine the range of validity of the vi
scocapillary model and find the low-flow limit outside this range. The low-
flow limit was determined here theoretically and experimentally. The 2-D Na
vier-Stokes equations with free surfaces describe liquid flow in the coatin
g bead. Theoretical approaches solve the Navier-Stokes system by either usi
ng Galerkin's method with finite-element basis functions or applying a long
-wave expansion. The minimum layer thickness at a set of parameters was det
ermined by the turning point on the solution path as the thickness was dimi
nished The minimum film thickness was measured experimentally by determinin
g the flow rate at which the coating bead breaks, leading to stripes of coa
ted and uncoated web. Results show that the low-flow limit of the coating b
ead at large Capillary and Reynolds numbers fundamentally differs from that
at their low numbers. At large Capillary and Reynolds numbers, the minimum
film thickness that can be coated decreases with increasing coating speed.
The coating window of the process is much larger than that in the literatu
re, broadening the applicability of this coating method.